1. Field of the Invention
The present invention relates to a hydraulic clutch for use in a vehicular automatic transmission or the like. Particularly, the invention is concerned with an improvement of a wave spring in a hydraulic clutch which wave spring is disposed between a hydraulic piston for engagement of a multiple disc clutch and a friction element such as a clutch plate to cushion a shock generated upon engagement of the clutch.
2. Description of the Prior Art
Heretofore, there has been known well a fluid type automatic transmission of the structure in which a friction engaging device such as a clutch or a brake is selectively engaged or released according to vehicular running conditions by means of a hydraulic control unit, whereby the power of an input shaft is transmitted to an output shaft while changing the change gear ratio through an auxiliary speed change mechanism such as a planetary gear for example.
A hydraulic clutch which constitutes part of such a friction engaging device will now be described with reference to FIGS. 7 and 8 in connection with such prior art as described in Japanese Utility Model Laid Open No. 68631/91 for example. The hydraulic clutch comprises a drive-side clutch drum 01, clutch plates 02 splined to the inside of the clutch drum, multiple-disc type clutch discs 03 splined to a driven-side clutch hub (not shown) and arranged alternately with the clutch plate 02, and a clutch piston 04 for engaging or releasing the clutch plates 02 and clutch discs 03.
When a clutch oil pressure is introduced into a hydraulic oil chamber 06 of the clutch piston 04, the clutch piston operates to engage the clutch, while upon relief of the oil pressure the clutch piston 04 is pushed back by a return spring (not shown) to release the clutch.
In this type of a hydraulic clutch, a dish plate 07 is usually disposed between a pressing surface 05 of the clutch piston 04 and the clutch plate 02 located at the frontmost position to cushion the shock generated upon engagement of the clutch. The reference numeral 08 in the figure denotes a return spring retainer.
The known dish plate 07 is generally of such a construction as shown in FIG. 8. For example, the dish plate 07 is fabricated by cutting out a ring-shaped plate portion 09 from a flat plate and then machining it into a dish-like form. Therefore, the yield from the flat plate is poor, resulting in that the component in question becomes expensive despite the simple structure thereof.
For solving the above-mentioned problem it is suggested to substitute the above dish plate by a so-called wave spring. The wave spring is fabricated by forming a single elongated strip-like spring material into a ring shape with use of a roll and at the same time waving the ring in its circumferential direction. A shock induced by engagement of the clutch upon operation of the clutch piston is cushioned by deformation of such a ring-like wavy spring.
However, since this wave spring comprises a ring-like spring member having certain width and thickness and an arbitrary number of wavy portions (portions bent in the rotating axis direction) in the circumferential direction, one of upper and lower peaks of each wavy portion of the wave spring contacts the working surface 05 of the clutch piston 04, while the other peak contacts the surface of the frontmost clutch plate 02. These contacts assume a so-called linear contact at the time of release of the clutch or at the beginning of engagement of the clutch. With movement of the clutch piston 04, the spacing between both contacts becomes narrower, and the shock induced upon engagement of the clutch is cushioned by a compressive deformation of the wave spring.
However, even such a wave spring involves a drawback. More particularly, when the clutch is released, the dish plate and the wave spring generally create a so-called co-rotation with rotation of the clutch plate which is rotating continually. Unlike the contact throughout the whole surface like the dish plate, the contact between each peak of the wave spring and the surface of the clutch plate or the clutch piston is generally a linear contact as mentioned above, so the co-rotation of the wave spring occurs while a wave peak of the wave spring is in sliding contact with the working surface of the clutch piston which is off.
In the case where the working surface is an iron surface as in the conventional clutch piston, there occurs no special inconvenience even upon rotation in sliding contact between a wave peak of the wave spring and the working surface. However, in the case where the working surface is of a soft material as in an aluminum clutch piston for example, there occurs a marked wear on the working surface of the clutch piston due to friction caused by a wave peak of the wave spring.